1. Field of the Invention
This invention relates to a light quantity adjusting device to be used for a video camera or a still picture camera or more broadly to an optical apparatus such as a camera or the like.
2. Description of the Related Art
The video cameras and the still-picture film using cameras of these days are provided with automatic focusing devices and automatic light quantity adjusting devices such as automatic iris devices (hereinafter referred to as light quantity adjusting device). Generally, many of these light quantity adjusting devices are constructed on the basis of a closed-loop control system. The automatic iris device which is included in the video camera is also constructed on the basis of the closed-loop control system. FIGS. 4, 5 and 8 of the accompanying drawings show the control system of the conventional automatic iris device.
Referring to FIGS. 4 and 8, the rotor 4 of a motor part 1 is arranged to drive the diaphragm blades of the automatic iris device. A driving coil 5 is provided for rotating the rotor 4. A power amplifying circuit 3 is provided for controlling the current applied to the driving coil 5. A damping coil 6 is provided for detecting the rotating speed of the rotor 4. A speed signal amplifying circuit 7 is arranged to amplify the speed detection output of the coil 6. A light quantity control signal input terminal Q is arranged to receive a light quantity control signal which is, for example, a luminance signal included in a video signal. A comparison computing circuit 2 generates an output which is in proportion to a difference between a speed control signal output from the amplifying circuit 7 and the light quantity control signal. The driving coil 5, the rotor 4 and the damping coil 6 are included in the motor part 1.
The conventional automatic iris device is arranged to have the output of the coil 6 amplified by the speed signal amplifying circuit 7 and to have the amplified signal negatively fed back to the comparison computing circuit 2 as a speed control signal.
Referring to FIG. 5, a base plate 100 has an aperture 101 which allows a light flux to pass therethrough and a support part 102 on which the rotor 4 and the coils 5 and 6 are mounted. A driving lever 105 is secured to the rotating shaft of the rotor 4 and is arranged to swing accordingly as the rotor 4 rotates. A spring is arranged to constantly urge the driving lever 105 to move in the direction of closing the aperture. Diaphragm blades 106 and 107 are arranged to adjust the quantity of light by varying the area of aperture in a state of having apertures 106a and 107a coinciding with each other. A retaining plate 110 has an aperture 110a formed therein in such a way as to allow the light flux to pass therethrough.
The diaphragm blades 106 and 107 are slidably carried respectively between the base plate 100 and the retaining plate 110. The pins 105a and 105b of the driving lever 105 are inserted into slots 106b and 107b. This arrangement enables the swinging movement of the driving lever 105 to cause the diaphragm blades 106 and 107 to slide to vary the diaphragm aperture which is formed jointly by their apertures 106a and 107a.
The conventional automatic iris device which is arranged in the above-stated manner, however, has presented the following problems which must be solved:
(i) The device necessitates the provision of the speed detecting damping coil 6, which occupies a relatively large space. This not only increases the size of the device but also prevents reduction in cost.
(ii) The damping coil 6 and the driving coil 5 are disposed close to each other. Therefore, a change in a current applied to the driving coil 5 brings about a current at the coil 6 due to mutual induction. This current lowers the accuracy of the rotor speed detection signal produced by using the coil 6. Since the detection signal is negatively fed back to the driving coil 5, the control over the speed and the position of the rotor 4 tends to become unstable.